AMS 6509 and AMS 6517 are high hardness, temper resistant alloy steels designed for use in metal to metal contact applications such as, but not limited to, gears, bearings and shafts. Due to their high hardness after heat treatment (HRC 60-64), any metal to metal contact surface must be very smooth so that the designed for surface durability performance of the metal to metal contacts can be achieved, especially if the AMS 6509 or AMS 6517 is mated to a lower hardness alloy component.
Typically, AMS 6509 and AMS 6517 are machined after heat treatment to produce the necessary final dimensions and geometry of the metal to metal contact surfaces. Machining techniques such as, but not limited to, turning, broaching, grinding, skiving or honing can be employed to produce these metal to metal contact surfaces. Because of some of the material properties of AMS 6509 and AMS 6517, such as the high surface hardness after heat treatment, these alloys are susceptible during grinding or other surface machining process, to forming a distressed material layer consisting of one or all of the following:                a) White layer, which is a brittle, hard surface layer prone to cracking. This is material that has been melted and rapidly solidified. The white layer is densely infiltrated with carbon and has a distinct separate structure to that of the parent metal. The amount of white layer can be the general machined surface area or it can be small, isolated areas of the machined surface. White layer is typically very thin from machining operations, often on the order of 1-4 microns deep. White layer is typically discontinuous in that it is intermixed with non-distressed metal.        b) Strain hardened surface zone whereby the metal undergoes changes in its atomic and crystalline structure, resulting in increasing resistance of the metal to further deformation. The strain hardened surface zone is higher in hardness than the surrounding area. The amount of strain hardened zone can be the general machined surface area or it can be small, isolated areas of the machined surface. Strain hardening from machining operations is typically very thin, often on the order of 1-4 microns deep. Strain hardened surface zones are typically discontinuous in that they are intermixed with non-distressed metal.        c) Oxidization resistant material zone whereby the machined surface is in a conversion coating reaction inert condition. The amount of oxidization resistant surface area can be the general machined surface or it can be small, isolated areas of the machined surface. The oxidization resistant material layer is typically very thin, often on the order of 1-4 microns deep. Oxidization resistant material transformed zones are typically discontinuous in that they are intermixed with non-distressed metal.        d) Temper burnt surface zone (also referred to as grind burn), is a soft layer caused by the alloy being heated above its tempering temperature and then slowly cooled. Temper burn is generally a large area and not small, isolated areas of the machined surface. Temper burn is generally deeper than the other three types of distressed layers, often on the order of 10-50 microns deep. Temper burn is typically continuous across the burnt zone and is not intermixed with non-distressed metal. Temper burn is considered to be a severe material degradation that often required the scrapping of the component.        
In the following, reference to susceptible high-hardness steel components is intended to denote components having surface hardness of greater than 60 HRC and which are susceptible to one or all of the above referenced distress conditions.
After final machining, components made of these AMS 6509 and AMS 6517 alloys often require inspection to confirm they are free of temper burn before they can be put into operation. Unfortunately, it has been found that traditional temper burn inspection techniques, such as 4% nitric acid in ethyl alcohol etch (Nital Etch), do not accurately predict the presence of temper burn on these alloys. An alternative method of temper burn inspection etching using 50% hydrochloric acid in water at 160° F. for 30 minutes has been developed. This alternative method is environmentally and health disadvantageous and does not always produce accurate predictions of the presence of temper burn.
It is believed that for accurate temper burn inspection of AMS 6509 and AMS 6517 via 4% nitric/alcohol etch or via 50% hydrochloric/water etch, or by other methods such as Barkhausen, the white layer, strain hardening layer, and/or the oxidization resistant material layer must be removed. It is also believed that the presence of a white layer, a strain hardened layer and/or an oxidization resistant material layer, if left on the surface by themselves, will reduce the surface durability performance in metal to metal contact components when put into operation.